Glass strands are commonly formed by attentuation of molten glass contained in a bushing through bushing tips located at the bottom of the bushing. The filaments drawn from the bushing tips are passed across the application surface of an applicator where they are coated with a binder and/or size. The filaments are then gathered into one or more unified strands by a gathering shoe, which is typically a grooved wheel or cylinder formed of materials such as graphite. The unified strand or strands traverse across the face of a rotating spiral and are collected in a criss-cross pattern as a forming package on a rotating collet.
Ideally, this process is virtually continuous, being stopped only for doffing a complete forming package and beginning the winding of a new one. However, this ideal is not always obtainable. For numerous reasons, including variations in bushing temperature, roughened surfaces of the spiral, applicator, or gathering shoe, flooding of the bushing and the like, breakouts of the glass strand can occur. When these breakouts do occur, glass beads may be formed under the bushing tips where the filaments are drawn. These beads are extremely hot and will burn the application surface of the applicator if they come into contact with it as they drop from the bushing. Since the application surface is typically formed of a material such as rubber and is smooth so that the filaments travel over it with minimum friction, any roughening of the surface caused by burning of the surface leaves a location on the surface for glass filaments to catch and again breakout. Thus a damaged applicator surface must be replaced, necessitating a time delay and the cost of replacement. In addition, any glass beads dropping from the bushing may enter the reservoir in the applicator containing the binder and/or size and contaminate this material or foul parts of the applicator. This can result in necessitating replacement of the binder and/or size and/or a clean up of the applicator.
Likewise, spiral damage may result from contact with the glass beads when a breakout of strand occurs. Should a glass bead come in contact with the spiral while it is rotating at high speed, this large bead, in relation to the size of the glass strand, may, due to the combined effects of the mass of the bead and the momentum of the rotating spiral, nick the surface of the rotating spiral. Any such nick is another location where strand may catch and again breakout when the process is resumed. Thus, as with the applicator, a time consuming and costly replacement of the spiral is often necessitated by its contact with glass beads.
Finally, the strand on the forming package, which is fragile and highly susceptible to damage by abrasion, will be harmed should it come into contact with glass beads falling from a bushing after a breakout as the winder is rotated. The bead damage is often severe enough that the entire forming package must be scrapped since the hot beads hitting a rotating package dissipate across the surface causing extensive damage across the length and circumference of the package.
It is desirable, therefore, to protect the glass fiber forming operation as much as possible from the harmful effects of a breakout.
In U.S. Pat. No. 3,560,178 an apparatus is disclosed for detecting the breakout of strand in a glass fiber forming operation. The apparatus includes a pivoted gathering shoe which is electrically connected to the power supply for the spiral and collet. The apparatus includes no means to protect the applicator from bead damage. More importantly, there exist problems with this structure. Since the rotation of the gathering shoe connects and disconnects the power supply to the collet and spiral, and since the gathering shoe may rotate while the strand is passing therethrough by the force exerted by the downwardly passing strand, power can be cut to the collet and spiral while strand is still passing through the gathering shoe without a breakout having occurred. In addition, the apparatus of this patent includes a liquid mercury switch. Such a switch is highly susceptable to vibration. Vibrations will occur in glass fiber formation, with the result that the mercury switch may trip itself and cut power to the collet and spiral, again without a breakout having occurred.
Further, this system is difficult to keep free of accumulated binder and/or size and it thus has a tendency to gum up. The result of this tendency to gum up is that the gathering shoe often fails to pivot properly upon a strand breakout and thus fails to protect the glass fiber forming apparatus.
It is desirable, therefore, to provide a strand breakout detection system of increased reliability over the prior art systems.